The present invention relates to a plastic injection nozzle assembly and in particular to a nozzle assembly for use in injecting both plastic and a gas into a mold cavity. Such a nozzle assembly can be used for injection molding of thermal plastic parts with hollow walls, hollow ribs or hollow parts with solid webs connecting the walls together.
Injection molding of hollow parts is accomplished by first injecting into a mold cavity a predetermined quantity of molten plastic which is less than the volume of the mold cavity. After the plastic is injected, a gas is injected into the molten plastic forcing the plastic through the mold cavity and holding the plastic against the wall of the mold cavity while the plastic cools. This aids the external surface of the plastic material in assuming the precise shape dictated by the mold surface. The gas pressure also assists in filling the narrow portions of the mold cavity with plastic which is often difficult to accomplish and can only be accomplished with high injection pressures in a solid plastic injection molding. The use of the gas pressure will reduce the tendency of the plastic to shrink away from the mold walls during cooling. In solid injection molding the plastic is kept under high pressure to prevent shrinkage during cooling by maintaining the plastic injection pressure. However, this produces unwanted stress in the molded part as well as significant stress in the injection molding machinery.
In gas injection molding, articles are produced by injecting molten plastic into the mold cavity and charging a body of pressurized gas therein to form a hollow portion in the thermal plastic material. The gas may be nitrogen, air or other gas that will not react with the polymer being injected. Pressure is maintained on the gas in the hollow space within the molded object until the plastic material in the mold cavity has set. Thereafter, the pressurized gas is released from the molded part hollow area and the molded part can be taken out of the mold cavity.
Several nozzle assemblies have been proposed for both gas and plastic injection. These nozzles, however, suffer from one or more of the following deficiencies. Some nozzles require that the nozzle assembly be physically removed from the mold in order to vent the mold interior to exhaust the gas from the molded part interior. This is particularly disadvantageous when multiple nozzles are required to mold large parts. Some nozzles are prone to plastic plugging of the gas passage near the tip of the nozzle assembly. In order to prevent this plugging, the gas passages are often made extremely small in size which decreases the gas flow rate into and out of the mold cavity. Another disadvantage is that the actuators used to move various internal components of the nozzle assembly are often off-set to one side of the nozzle center line. This can create uneven loading of the moving components resulting in binding of these components, increasing wear and nozzle maintenance.
It is an objective of the present invention to provide a combination gas and plastic injection nozzle assembly that overcomes the above mentioned deficiencies.
The combination gas and plastic injection nozzle of the present invention includes a tubular nozzle body that is tapered at one end to form a nozzle tip. The nozzle body forms an orifice at the tip through which the molten plastic is injected into the mold. The opposite end of the nozzle body is coupled to a conventional injection molding machine for receiving molten plastic resin. A hollow closure rod extends longitudinally through the nozzle body and serves as a plug that is movable into the nozzle body orifice to close the orifice, stopping the flow of plastic. The annular space between the exterior of the hollow rod and the nozzle body forms a flow passage for the molten plastic. The orifice is opened by retracting the nozzle rod into the nozzle body.
The interior of the hollow closure rod forms a passage for the injection gas. The end of the nozzle rod forms a small orifice through which the gas is injected into the mold after the nozzle rod has closed the nozzle tip to plastic. A second hollow rod, a gas rod, extends through the nozzle rod to supply the injection gas to the orifice at the end of the nozzle rod. The gas rod ends in a closed tip which is seated against an interior engagement surface of the nozzle rod to close the gas orifice. The gas rod is longitudinally movable away from the seat to initiate the flow of gas. Openings in the gas rod before the closed tip enable the gas to flow from the gas rod into the gas passage in the nozzle rod.
Both the nozzle rod and the gas rod are moved by actuating cylinders that are positioned in line with the axis of the nozzle body so as to avoid an off-set load of either rod as they are moved.
It is a further object of the present invention to provide an improved process utilizing a mold with multiple cavities and at least one nozzle assembly for each mold cavity.
The injection nozzle assemblies are particularly useful in a multiple nozzle injection molding process. Injection molding process efficiencies can be improved by providing a mold with multiple cavities to produce multiple articles with a single closing and opening of the mold. By using multiple nozzles with at least one nozzle per cavity within a mold assembly, a sequential process can be used in which the cavities are filled with plastic one at a time rather than all cavities being filled simultaneously. Such a sequential process has numerous advantageous over a simultaneous process using one nozzle connected to several mold cavities by a runner system. By separately injecting the resin into each cavity, the quantity of plastic to each cavity and the flow rate can be controlled individually for each cavity. Likewise, the subsequent gas injection into each cavity can also be independently controlled. A further advantage is that the mold clamping load can be significantly reduced since only one cavity is subjected to the plastic injection pressure at any given time. This enables lighter weight mold components to be used and also decreases the process time required to build up the necessary mold clamping force.
Further objects, features and advantages of the invention will become apparent from a consideration of the following description and the appended claims when taken in connection with the accompanying drawings.